Wireless mesh communication networks typically consist of a plurality of wireless routing nodes that operate in a peer-to-peer fashion to establish communication paths to one another for the purposes of providing network access to wireless clients or mobile stations. Some wireless mesh networks are hierarchical in nature with the routing nodes that bridge wireless traffic onto a wired network at the top of the hierarchy. The wireless mesh routing nodes can be one-, two-, or multiple radio systems including omni-directional and/or directional antennas, and systems that provide backhaul traffic over the same mesh hierarchy but over multiple channels. In one-radio systems, the radio unit is used for purposes of acting as an access point to its clients, as well as acting as a backhaul to a parent routing node. In two-radio systems, one radio unit typically provides access point service to wireless clients as well as child routing nodes, while the other radio unit is used as a backhaul to a parent routing node. Multiple radio designs typically dedicate one radio for access, one or more to service the backhaul, and may also dedicate a radio for the purposes of monitoring the RF environment and other conditions on multiple radio frequencies. In certain wireless mesh networks, the backhaul radio operates in ad-hoc station mode, appearing as a peer node to the parent routing node. Those radios in the network providing access to clients operate in access point mode, providing wireless connections to mobile stations.
As the number of routing nodes in a wireless mesh network increases, certain problems are created due to the fact that the routing nodes essentially share, and therefore compete for access to, the transmission medium. To avoid radio interference among the routing nodes, each routing node in a wireless mesh network may generally employ a packet collision avoidance mechanism as part of the wireless communications protocol, such as the 802.11 protocol. Accordingly, a typical way of initiating communication between routing nodes begins with the transmission of a “Request-to-send” (RTS) packet by an initiating routing node. This packet is typically received by all routing nodes within the transmission range of, and operating on the same channel, as the initiating routing node. The RTS packet notifies these routing nodes that the initiating routing node intends to transmit a flow of packets to a specified target routing node. After receiving an RTS packet, the target routing node responds by transmitting a “Clear-to-send” (CTS) packet that notifies the initiating routing node that the target routing node is ready to receive the data stream. The CTS packet also serves to notify other routing nodes within range that the transmission medium has been reserved such that they refrain from transmissions that might interfere with the transmission between the initiating and target routing nodes. Accordingly, since other routing nodes within range of the initiating and target routing nodes are forced to remain idle during transmission of the data stream, system throughput can be drastically impaired as the number of routing nodes and clients increase. Further, IEEE 802.11 or other collision avoidance mechanisms may not ensure that each routing node receives a fair share of access to the transmission medium.
To address these problems, mesh network routing nodes can employ channel assignment schemes and mechanisms to reduce or eliminate interference between adjacent routing nodes. The limited number of non-overlapping operating channels in a given band, however, does present certain limitations for channel re-use when the number and/or density of routing nodes increases. Directional antennas have also been deployed to reduce or control interference across routing nodes. Without some coordination mechanism, however, interference between routing nodes, as well as fair access to the transmission medium, remains a significant factor. In light of the foregoing, a need in the art exists for coordinating wireless transmissions across routing nodes in a wireless mesh network. Embodiments of the present invention substantially fulfill this need.